1. Field of the Invention
This invention pertains to the field of automatic transmission hydraulic controls and actuation systems. More particularly, the invention pertains to such a system that maintains pressure in the friction element, modulates the application of the element and applies the friction element at full torque capacity.
2. Description of the Prior Art
In an automatic transmission the hydraulic circuit that controls and actuates friction elements, such as clutches and brakes, contains a source of regulated line pressure which is opened and closed through various control valves to clutches and brakes that are engaged and released selectively to produce the various forward speed ratios and reverse drive. Generally when gear ratio changes are made, a shift valve, dedicated to controlling a gear ratio change, changes state due to the presence or absence of solenoid pressure applied to the shift valve in accordance with an electronic signal produced by a microprocessor in response to a gear ratio command produced by software stored in electronic logic accessible to the microprocessor. When the friction element is to be disengaged, the appropriate shift valve closes communication between the source of regulated line pressure and the friction element, generally by removing solenoid pressure from the shift valve. However, when the friction element is disengaged, that portion of the hydraulic circuit between the friction element and shift valve is susceptible to being filled with air. This condition delays reapplication of the friction element and can result in a harsh gear ratio change, perceptible to the vehicle operator.
To avoid abrupt gear ratio change, various means have been devised to prevent accumulation of compressible fluid in the hydraulic circuit leading to the various friction elements. For example, various orifices are located in the hydraulic circuit between the various friction elements in the high pressure line pressure source during the period when fluid is vented from the friction element and absent in the hydraulic circuit leading to the friction element. Orifices for this purpose allow pressurized fluid to bleed into the portion the hydraulic circuit leading to the friction element when the transmission is disposed for operating in the park range, reverse, neutral and certain forward drive ratios, other than the ratio during which the relevant friction element is engaged and supplied with pressurized fluid. Permitting fluid at line pressure to enter the hydraulic circuit through the orifices prevents to a certain degree harsh shift engagement when the friction element is connected directly to the fluid pressure source through the shift valve. However, the orifices must be sized such that they do not allow enough pressure to build up in the circuit to move the clutch piston of the friction element during those periods when the friction element is not engaged.